1. Technical Field
This invention relates to a ferroelectric thin film, especially a ferroelectric thin film formed on a semiconductor crystal substrate for use as electronic devices including dielectric memories and semiconductor devices such as non-volatile memories, infrared sensors, optical modulators, optical switches, and OEIC (opto-electronic integrated circuits), and a method for preparing the same. It also relates to electronic devices.
2. Background Art
Electronic devices have been elaborated which are fabricated by forming dielectric films on silicon substrates or semiconductor crystal substrates, followed by integration. Studies have been made to fabricate LSIs having a higher degree of integration and dielectric isolated LSIs relying on SOI technology, by combining semiconductors with dielectrics. Since non-volatile memories, infrared sensors, optical modulators, optical switches, OEIC (opto-electronic integrated circuits) or the like can be fabricated using ferroelectrics which are one class of dielectrics, active research works have been made on the ferroelectric thin film material. Non-volatile memories can be implemented by combining the polarization inversion phenomenon of ferroelectrics with semiconductor devices.
The ferroelectric materials which are currently under consideration because of their superior polarizability include lead family oxide thin films such as PbTiO.sub.3, PZT, and PLZT and bismuth family oxide thin films such as Bi.sub.2 Ti.sub.2 NbO.sub.9. These thin film materials must be crystallized in order to exhibit ferroelectric characteristics. For crystallization, a technique of raising the temperature during thin film formation to 600.degree. C. or higher or a technique of annealing at 600.degree. C. or higher after thin film formation is described in Jpn. J. Appl. Phys., 31, (1992) 3029, Jpn. J. Appl. Phys., 33, (1994) 5244, and Mat. Res. Soc. Sympo. Proc., 310, (1993) 473. Lead and bismuth oxides, however, are difficult to control their composition since they have a high vapor pressure and can evaporate during heat treatment at high temperature to induce a compositional deviation. The ferroelectric thin film material should, of course, have superior polarization characteristics, but is also required to have stability at high temperature in that oxides of constituent elements of the ferroelectric material have a low vapor pressure when the reproducibility and mass producibility of semiconductor devices such as memories which are fabricated by applying a semiconductor process to the thin film material are taken into account.
Also devised was a memory of the structure using ferroelectric material in the gate of FET which is one of non-volatile memories. As described in a technical report issued by the Japanese Electronic Information Communication Society, SDM 93-136, ICD 93-130, (1993-11), page 53, the memory using ferroelectric material in the gate has not reached the practically acceptable level because there remain many outstanding problems associated with their manufacture and the physical properties of ferroelectric thin films. For this type of memory, it is ideal, but difficult to implement a metal-ferroelectric-semiconductor (MFS) structure in the memory cell and therefore, a metal-ferroelectric-insulator-semiconductor (MFIS) structure or metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure must be fabricated. In order that the ferroelectric material undergoes polarization inversion to ensure memory operation for this structure, a sufficient electric field must be applied across the ferroelectric material. Since the ferroelectric material and insulator are formed in series as capacitors in the MFIS and MFMIS structures, it is necessary to take appropriate measures for lowering the dielectric constant of ferroelectric material and raising the dielectric constant of insulator in order that a sufficient electric field be applied across the ferroelectric material. Among the ferroelectric materials which have been heretofore investigated, lead family oxide thin films such as PbTiO.sub.3, PZT, and PLZT have a dielectric constant as high as 400 to 1,000 or more. There is a desire to have a ferroelectric thin film having a low dielectric constant.
It is also desired to use a single crystal as the dielectric material in order to ensure optimum device characteristics and reproducibility thereof. Polycrystalline material is difficult to provide satisfactory device characteristics due to disturbance of physical quantities by grain boundary. This is also true for thin film materials and a dielectric epitaxial film which is as close to a complete single crystal as possible is desired. In order to implement a dielectric epitaxial film in the above-mentioned MFIS structure or MFMIS structure, a metal and a ferroelectric material must be epitaxially grown on a single crystal silicon substrate which is a semiconductor substrate, which has not been accomplished.
Furthermore, although the MFMIS structure needs a satisfactory metal electrode film, a single crystal silicon substrate having a conductive epitaxial film of (111) orientation, for example, is not available in the prior art.
Among conventional lead and bismuth family materials, there are available no thin films free of a compositional deviation and closer to a single crystal. High reactivity with silicon serving as the substrate allows for diffusion of Pb or Bi into the silicon substrate, which has serious influence on the characteristics of integrated circuits fabricated in the silicon substrate. Therefore, there is a desire to have a ferroelectric thin film material other than the lead and bismuth families. For the above-mentioned application as a memory of the structure using ferroelectric material in the gate of FET, there are available at present no ferroelectric thin film materials having a low dielectric constant.